Composition comprising epoxy resin, copolymer of butadiene and acrylic acid, curing agent and inorganic metal salt

ABSTRACT

1. A composition of matter comprised of an epoxy resin, a copolymer of butadiene and acrylic acid, a curing agent, and between about 2.5 and about 30 percent by weight of an inorganic metal salt selected from the group consisting of metal chromates, metal bichromates, metal phosphates, and mixtures thereof.

This invention relates to improvements in rocket motors and other gasgenerating devices which utilize solid propellants. More particularly,this invention relates to improvements in bonding characteristicsbetween the insulating liner of rocket motor combustion chamber wallsand the propellant charge contained within the chamber.

Rocket motors are generally comprised of a suitable vessel having anozzle in one end thereof, a combustion chamber containing a propellantcharge, and an ignition system capable of igniting the propellant chargewhen desired. Control of the thrust throughout the combustion period ofthe propellant may be obtained by shaping of the propellant to giveburning surfaces that will yield the desired thrust program during thecombustion period. The use of propellants in the form of a rod, rod withinternal star, hollow rod, multiple port rod, and the like, for thispurpose is well known in the art. In addition, certain desired surfacesof the propellant may be coated with a suitable inhibitor to suppressburning of these surfaces, thereby providing another means ofcontrolling the thrust. One method of inhibiting the burning surface ofthe propellant comprises coating the interior surface of the combustionchamber, after suitable surface preparation, with an insulating binder,and the exterior periphery of the propellant, by casting or otherwise,is then secured to the interior combustion chamber surfaces by means ofthe insulating binder. The insulating binder may be used for severalpurposes. It may serve as an insulator for the rocket vessel to protectit against the high temperatures generated during combustion. It mayserve as a binder to secure the propellant to the wall of the combustionchamber. In addition, it may be used to inhibit burning of the outerperiphery of the propellant. Considerable difficulty has beenencountered in obtaining a satisfactory liner material that will serveall three purposes. Various elastomeric materials have been used as theinsulating liner, but the bond between the liner and propellant oftenfails if the rocket is stored for extended periods after fabrication orif the rocket is subjected to extreme variations in temperatureconditions prior to or during firing. Therefore it is necessary to firesuch rockets very soon after fabrication, and expensive and cumbersomemeans must be employed to maintain the temperature of the rocketrelatively constant prior to firing. Unless these precautions are taken,there may be failure of the bond between the liner and propellant, whichresults in uncontrolled burning of the propellant during the combustionperiod. Such uncontrolled burning not only causes undesired variationsin the thrust program, but also may cause rupture and/or explosion ofthe rocket vessel. In addition, when nitroglycerine is employed as acomponent of the propellant, there is a migration of the nitroglycerinethrough the propellant and liner to the chamber wall, which often causesfailure of the bond between the liner and wall, thereby resulting inuncontrolled burning of the propellant.

It is a primary object of this invention to overcome the disadvantagesinherent in conventional techniques for bonding rocket motor liners androcket motor propellant charges.

A further object of this invention is to provide an improved rocketmotor.

Still another object of this invention is to provide an insulating linerfor the combustion chamber of rocket motors having improved bondingproperties.

It is another object of this invention to provide an improved processfor preparing rocket motors.

Still a further object of this invention is to provide a rocket motorcombustion chamber having an insulating liner bonded to the propellantcharge, wherein the bond between the liner and the propellant chargeresists deterioration during extended periods of storage.

Another object of the invention is to provide a novel rocket motor linerwhich remains firmly bonded to the propellant when subjected to extremevariations in temperature.

It is a further object of the invention to provide a novel insulatingliner for rocket motors which inhibits migration of nitroglycerine frompropellants containing it.

These and other objects of the invention will be apparent from thefollowing detailed description thereof.

It has now been discovered that improved bonding between a rubbery linerof a rocket motor combustion chamber and a solid propellant charge isreadily obtained by coating the interior surfaces of the combustionchamber with a layer of an uncured fluid mixture capable of being curedand solidified to a material having rubbery properties, the fluidcontaining between about 2.5 and about 30 percent by weight of aninorganic metal salt. After curing and solidification of the rubberyliner a thermoplastic propellant charge is then cast into the linedportion of the chamber, and upon solidification of the propellantcharge, a strongly adhering bond is formed between the rubbery liner andthe propellant charge.

More in detail, rocket motor vessels, gas generator vessels, and thelike, constructed of a suitable material such as steel alloy, stainlesssteel, aluminum alloy, low carbon steel, ceramic lined steel, moldedepoxy resin-coated glass fiber, and the like, may be treated inaccordance with the technique of the instant invention. For purposes ofclarity, the invention will be described as applied to rocket motorvessels, but it will be recognized by those skilled in the art that theinvention is applicable to gas generators, and other devices whichemploy solid propellant. The interior surfaces of the cumbustion chamberare cleaned by sanding and/or applying cleaning solvents to remove rust,grease and the like, to improve bonding between the interior combustionchamber surface and the liner.

The liner is formed from an uncured fluid mixture containing aninorganic metal salt, the mixture being capable of being cured andsolidified to a material having rubbery properties. It is preferred toemploy a mixture of an inorganic metal salt, an epoxy resin and arubber-base material with conventional curing agents, since the epoxyresin component not only enhances the bonding strength of the liner butalso impedes the migration of nitrogelycerine from the propellant to thecombustion chamber surfaces. However if the propellant does not containnitroglycerine, satisfactory results can be obtained by omitting theepoxy resin component.

The words "liquid" and "fluid" when used to define the uncured fluidmixture capable of being cured and solidified to a material havingrubbery properties throughout the description and claims are intended toinclude solutions, dispersions, slurries or paste-like fluids having theconsistency of conventional rubber-base cement.

Rubber-base materials suitable for use in preparing the liner of theinstant invention include fluid polymers or copolymers of one or morerubber-forming diolefins, such as butadiene and including substittutedbutadienes such as isoprene, chloroprene, dimethyl butadiene, methylpentadiene, and the like, with or without other suitable copolymerizablematerials such as acrylic acid, styrene vinyl esters, acrylic esters,methacrylic esters, and the like. Liquid dispersions or sulutions ofnatural rubber, C rubber, nitrile rubber, GRS rubber, neoprene,polysulfide rubber, cellulose acetate, and the like, which may be curedand solidified by the application of a curing agent and/or heat may beemployed.

Curing agents for these rubber-base materials are well known in the art,and will vary with the type of rubber base material employed. Forexample, magnesium oxide, diphenylguanidine, p-quinonedioxime,peroxides, sulfur, and the like, are suitable curing agents.

The epoxy component of the rubbery liner of the present inventioncomprises liquid monomeric or polymeric polyepoxides containing aplurality of epoxide groups having the formula ##EQU1## At least one ofthese groups in the polyepoxides is terminal. The polyepoxides may besaturated or unsaturated aliphatic, cycloaliphatic, aromatic orheterocyclic and may carry non-interfering substituents, for example,chlorine, hydroxyl or ether groups. The polyepoxides are commonly formedby any of several reactions. One type by the reaction of aromatic oraliphatic polyhydroxy compounds with epichlorohydrin. Also useful inplace of epichlorohydrin are 3-chloro-1,2-epoxybutane, 3-bromo-1,3-epoxyhexane, 3-chloro-1,2-epoxyoctane and the like. To form the epoxyresins, the epoxide is reacted with an aliphatic or aromatic polyhydriccompound, for example, bisphenol A, resorcinol or polynuclear phenols,for example, 4,4'-diohydroxybenzophenone or bis(4-hydroxyphenyl) ethane.Sorbitol, glycerol and pentraerythritol are examples of suitablepolyhydroxy aliphatic compounds. Any of the commercially available epoxyresins are suitable for use in the present compositions including theresins sold under the names of Epon, Araldite ERL and Epi-Rez. Epon 828is an epoxy resin formed from bisphenol A and epichlorohydrin and hasaverage molecular weights of 350 to 400 and viscosities of 5,000 to15,000 centipoises at 25°C. (Gardner Holdt). Epon 834 is similar to Epon828 but has molecular weights of about 450 and viscosities between A₁and A₂ (Gardner Holdt). Araldite 6010 is similar to Epon 828 but has aviscosity of 16,000 centipoises. ERL is similar to Epon 828 but hasviscosities of 10,500 to 19,500 centipoises. Epi-Rez 510 is similar toEpon 828 but has viscosities of 9,000 to 18,000 centipoises.

Other suitable epoxies include the Oxiron, peracetic and Novolac types.The Oxiron resins are epoxidized aliphatic polyolefins which containepoxy groups terminally and along the carbon chain. The peracetic resinsare obtained by epoxidation of olefins by oxygen and selected metalcatalysts or by peracetic acid. Unox Epoxide 201 is abicyclo-diepoxycarboxylate, Unox Epoxide 207 is bicyclopentadienedioxide and Unox Epoxide 206 is vinylcyclohexene dioxide. Novolac resinDER is an epoxidized polynuclear polyhydric phenol. Additional dataconcerning these resins appears in Lee and Neville, "Epoxy Resins, TheirApplication and Technology," McGraw-Hill Book Company, Inc. New York,1957.

Inorganic metal salts suitable for use in the instant invention includemetal salts of chromates, bichromates and phosphates. Typical examplesof suitable metal salts include zinc chromate, zinc phosphate, leadphosphate, barium chromate, magnesium chromate, sodium phosphates,potassium phosphates, mixtures thereof, and the like. The metal saltsare used in finely divided form, for example, particles which pass a 100mesh screen, in order to obtain adequate dispersion in the rubber basematerial. Zinc chromate appears to yield a better bonding liner than theother metal salts tested.

The uncured fluid mixture used in the preparation of the liner isprepared by admixing the epoxy resin, rubber-base material, inorganicmetal salt and curing agent to yield a substantially homogeneousmixture. The proportion of metal salt is between about 2.5 and about 30percent by weight, the proportion of curing agents is between about 5and about 20 percent by weight and the proportion of combined epoxyresin and rubber-base material is between about 50 and about 90 percentby weight of the mixture. The weight ratio of rubber-base material toepoxy resin in the mixture is preferably between about 1:1 and about5:1, but the epoxy resin component may be omitted under certaincircumstances, if desired, as discussed above. The ratio of rubber-basematerial to epoxy resin should not be less than about 1:1, since the useof excessive epoxy resin may eliminate most of the desirable elasticproperties of the liner.

The uncured fluid mixture capable of being cured and solidified to amaterial having rubbery properties, with or without an epoxy component,as the case may be, is applied to the interior surfaces of thecombustion chamber by any suitable means, such as by brushing, byapplying with a spatula, and the like. If desired the mixture may beformed or mixed with a suitable solvent such as ethyl acetate, acetone,hexane, then the solution of organic material is sprayed onto theinterior surfaces of the combustion chamber, and the solvent isevaporated to yield a rubbery liner. In one embodiment of the invention,the cylindrical rocket motor vessel is placed on motor driven rollers orother suitable apparatus to rotate the rocket motor at a rate sufficientto develope the necessary centrifugal force to maintain a substantiallyuniform thickness of the layer of uncured fluid mixture on the interiorof the vessel until it is cured sufficiently to retain its layer formwithout rotation. Application of the lining material and rotation iscontinued until the desired thickness of rubber-base liner is obtained.The thickness of the rubber-base lining is preferably between about 1/32and about 1/4 inches, but any suitable thickness that will impart thedesired degree of insulation to the combustion chamber vessel may beemployed.

If desired, several layers of rubber-based material and epoxy resin maybe employed to form the liner. For example, the combustion chamber wallis coated with a layer of the above defined uncured mixture, and aftercuring, a thin layer of epoxy resin is applied. After curing, anotherlayer of the fluid uncured mixture is applied. Such a composite liner isparticularly suitable for inhibiting the migration of nitroglycerinefrom propellants containing it.

After applying the fluid uncured mixture to the combustion chambersurfaces in the manner described above, solidification of therubber-base liner is completed, due to the effect of the curing agent,with or without the application of heat. It is preferred to employ acuring agent and complete curing of the liner at ambient temperature fora period at least about 12 hours and as long as about 10 days or more.However solidification of the liner may be effected at elevatedtemperatures, for example, up to about 80°C., in less than about 6hours.

Rocket motor casings lined in accordance with the technique of theinstant invention may be stored for extended periods, for example, aslong as 90 days before adding the propellant charge, without adverselyaffecting the bonding characteristics between the liner and propellant.

Any thermoplastic propellant charge may be employed that is capable ofbeing poured and cast inside the combustion chamber of the rocket motorand subsequently solidified by curing. Suitable propellant chargesinclude the rubber-based and asphalt base propellants containingoxidizers such as ammonium perchlorate, potassium perchlorate, ammoniumnitrate, and the like; propellants such as nitrocellulose,nitroglycerine, cyclonite, pentaerythrite tetranitrate and mixturesthereof; and metal additives such as powdered aluminum, boron, copperand mixtures thereof. A typical analysis of a suitable thermoplasticpropellant composition is as follows:

    Component        Parts by Weight                                              ______________________________________                                        Nitrocellulose   58.6                                                         Nitroglycerine   24.2                                                         Dimethyl Phthalate                                                                             9.6                                                          Dinitrotoluene   6.6                                                          Ethyl Centralite 1.0                                                          Carbon Black     0.1                                                          ______________________________________                                    

Other suitable solid propellant compositions such as double basepropellants, composite double base propellants and those set forth inRocket Propulsion Elements, by George P. Sutton, published by John Wileyand Sons, Inc., Second Edition (1956), may be employed.

The shape of the propellant may be in the form of a rod, hollow rod,star, etc., or other desired shape that can be cast in accordance withprior art techniques.

After solidification of the propellant charge a suitable closure meanshaving a nozzle for discharging combustion gases is then secured to theopen end of the chamber. A suitable ignition means is also provided inthe conventional manner.

When an inorganic metal salt is employed as a component of therubber-base liner and a propellant charge is cast into the liner inaccordance with the technique of the instant invention, an exceptionallystrong bond is obtained between the rubber-base liner and the propellantcharge, and substantially no rupturing of the bond is effected duringstorage, during extreme temperature variations or when the propellantcharge is ignited. As a result, substantially uniform burning anduniform thrust are obtained throughout the entire combustion period. Inaddition, it has been found that rocket motors prepared in this mannercan be stored indefinitely without any significant deterioration in thebond between the rubber-base liner and the propellant charge.Furthermore, when subjected to extreme temperature variations, forexample, cycling tests in which the rocket motors are stored at atemperature of -40°C. for 24 hours, then removed and immediately placedin an atmosphere at 60°C., and maintained in this atmosphere for 24hours, and then returned to storage at -40°C., the rocket motors of theinstant invention resisted bond failure after being subjected to morethan 10 cycles.

The technique of the instant invention is particularly effective whenused in combination with the technique of my copending application Ser.No. 143,275, filed of even date herewith, wherein a layer of the uncuredfluid mixture is applied to the combustion chamber surfaces, and beforesolidification, powder grains are partially embedded in the exteriorsurface of the layer. The mixture is cured and solidified, and athermoplastic propellant is then cast into the resulting rubber-baseliner having powder grains embedded therein.

The following examples are presented to illustrate the invention morefully without any intention of being limited thereby. All parts andpercentages are by weight unless otherwise specified.

EXAMPLE I

A rubber-base lining composition was prepared as follows: 35 parts of anepoxy resin having a molecular weight of about 375 formed by reactingepichlorohydrin with bisphenol A, were admixed with 5 parts of powderedzinc chromate until substantially homogeneous. To this mixture wereadded 52.3 parts of a copolymer of butadiene and acrylic acid, andmixing was continued until a substantially homogeneous mass was formed.A curing composition was prepared by admixing 5 parts ofp-quinonedioxime, 0.2 parts diphenylguanidine and 2.5 parts magnesiumoxide, and the resulting homogeneous curing composition was then admixedwith the resin-rubber-base-zinc chromate mixture until substantiallyhomogeneous. All mixing was carried out at room temperature.

A tensile testing apparatus was constructed which was comprised of twosquare steel plates, having a surface area on each face of 2.5 in.²,each plate being secured on one face to a pivot arm, which wasoperatively connected to a tensile strength measuring device. Theunattached faces of the two plates are cemented together to form a"sandwich" of steel plate/rubber-base liner/thermoplastic propellant/rubber-base liner/steel plate. The pivot arms then force the steelplates apart, the force necessary to cause rupture of the "sandwich"being measured in pounds per square inch.

A portion of the fluid or rubber-base composition, prepared as describedabove, was applied to the open face of each steel plate with a brush toform a layer of substantially uniform thickness (about 1/32 inch).

The rubber-base liner was cured at ambient temperature for 1 day. Thetwo plates with rubber-base liner were then cemented together with a 1/2inch thickness of a thermoplastic composite double base propellantcontaining nitroglycerine, nitrocellulose, ammonium perchlorate,powdered aluminum and additives. The cementing was effected by placingthe two lined plates vertical in a parallel position and 1/2 inch apart,then taping the bottom and two adjacent sides with a double thickness ofmasking tape to form a mold. The fluid propellant was then poured intothe top of the mold to fill it, and the taped plates with propellantwere heated to a temperature of 60°C., for about 24 hours to effectcuring of the propellant. The sandwiches were then cooled to ambienttemperatures and the tape was removed.

After curing of the "sandwich," the plates were pulled apart in thetensile strength testing apparatus. The maximum tensile strength andposition of the break were noted. The procedure was repeated for twoadditional "sandwiches" prepared in the same manner. The average tensilestrength of the three specimens was 77 pounds per square inch, and ineach test, the break in the sandwich occurred through the propellant,thus demonstrating that the bond formed by the technique of the instantinvention between the liner and propellant was stronger than the bond ofthe propellant alone.

For purposes of comparison, the procedure was repeated, with theexception that no liner was provided on the plates and the propellantwas bonded directly to the metal surface. The average tensile strengthof the three specimens was only 55 pounds per square inch, and in eachcase the break occurred at the metal-propellant interface.

EXAMPLE II

An uncured fluid mixture was prepared similar to that of Example I withthe exceptions that about 1 part of a rubber antioxidizing agent,(symetrical di-betanaphthyl-para-phenylenediamine) was admixed with thehomogeneous mixture, and 7.5 parts of zinc chromate were used instead of5 parts.

Three test sandwiches were prepared as in Example I with the exceptionthat the liner material was applied with a spatula, and the liner wascured for 7 days. The average tensile strength of the three specimenswas 99 psi., and break occurred through the propellant in each instance.

EXAMPLE III

An uncured fluid mixture was prepared in a manner similar to Example I,with the exception that 10 parts of zinc chromate instead of 5 partswere employed. The lining composition was applied to the plates with aspatula and was cured at ambient temperature for 13 days.

The lined plates were cemented together with propellant as in Example Iand tested in the tensile strength testing device. The average tensilestrength for the three specimens was 96 psi. and the break occurredthrough the propellant in each instance.

EXAMPLE IV

A rubber-base lining composition was prepared as follows:

30 parts of an epoxy resin having a molecular weight of about 375 formedby reacting epichlorohydrin with bisphenol A, were admixed with 15 partsof powdered zinc chromate until substantially homogeneous. To thismixture were added 47.3 parts of a copolymer of butadiene and acrylicacid, and mixing was continued until a substantially homogeneous masswas formed. A curing composition was prepared by admixing 5 parts ofp-quinonedioxime, 0.2 parts diphenylquanidine and 2.5 parts magnesiumoxide, and the resulting homogeneous curing composition was then admixedwith the resin-rubber-base-zinc chromate mixture until substantiallyhomogeneous. All mixing was carried out at room temperature.

The steel plates of Example I were coated by applying the liningmaterial with a spatula and the liners were cured for 17 days. Three"sandwich" specimens were prepared from thermoplastic propellant as inExample I, and after curing, the average tensile strength was 77 psi.The break in the specimens occurred through the propellant in eachinstance.

EXAMPLE V

A 3 pound test rocket motor vessel having a combustion chamber length ofabout 31/2 inches and an inside diameter of about 5 inches was cleanedby sandblasting and then lined with a 1/8 inch layer of the liningcomposition of Example I. The propellant of Example I was cast in theliner after curing of the liner and then heated to a temperature ofabout 60°C. for about 24 hours to effect curing of the propellant. Theresulting rocket motor was subjected to cycling tests at -40° and 60°C.for 24 hour periods, and after 16 cycles, no deterioration of theliner-propellant bond was found. Several rockets prepared in this mannerwere successfully fired after 3 to 6 temperature cycles.

EXAMPLE VI

A 70 pound JATO rocket was prepared, employing the liner composition ofExample III. No deterioration of the liner-propellant bond was foundafter 5 cycles between temperatures of -40° and 60°C. for 24 hourperiods.

EXAMPLE VII

Liners prepared in accordance with the instant invention have relativelyuniform elastic properties over a wide range of temperatures. A linerprepared as in Example I was formed into a sheet of 1/4 inch thicknessand cured. Several "dumbbells" were cut from this sheet, which wereabout 5 inches long, having a narrow portion in the center about 1/4inch wide, and each end being about 1 inch in width. The dumbbells wereplaced in a conventional stress testing machine to determine thepressure necessary to break the specimen and the percent of elongationbefore break. Three specimens were tested at -60°C., three at 25°C., andthree at 70°C. The average stress and per cent elongation at eachtemperature, was as follows:

    Temperature                                                                              Stress        Elongation                                           ______________________________________                                        -60°C.                                                                            396 psi.      39.6 per cent                                        25°C.                                                                             173 psi.      36.1 per cent                                        70°C.                                                                             126 psi.      32.0 per cent                                        ______________________________________                                    

For purposes of comparison, another liner sheet was prepared, in asimilar manner with the exception that iron oxide was substituted forthe zinc chromate.

This liner was tested in the same manner and the average stress to breakand per cent elongation at the break were as follows:

    Temperature                                                                              Stress        Elongation                                           ______________________________________                                        -60°C.                                                                            317 psi.      43   per cent                                        25°C.                                                                             157 psi.      34.75 per cent                                       70°C.                                                                             113 psi.      25.5  per cent                                       ______________________________________                                    

Thus it can be seen that greater pressure was required to break thespecimens containing zinc chromate and the elasticity was more uniformover a wide temperature range than a liner that did not contain zincchromate.

It will be recognized that many modifications and variations, some ofwhich are discussed above, will naturally present themselves to thoseskilled in the art without departing from the spirit of this inventionor the scope of the appended claims.

Having thus described the invention, what is claimed is:
 1. Acomposition of matter comprised of an epoxy resin, a copolymer ofbutadiene and acrylic acid, a curing agent, and between about 2.5 andabout 30 percent by weight of an inorganic metal salt selected from thegroup consisting of metal chromates, metal bichromates, metalphosphates, and mixtures thereof.
 2. The composition of claim 1 whereinsaid inorganic metal salt is zinc chromate.